In vitro colony formation (CFU-c) was studied in 7 patients with severe aplastic anemia (SAA) in complete autologous hematologic reconstitution. Colony formation was depressed in all patients (less than 5% of normal). Further studies showed that 1) patients' colony formation could be enhanced by removing T cells from the marrow, but not by removing adherent cells (AC); 2) patients' marrow cells showed a marked CFU-c inhibition against normal allogeneic marrow; 3) CFU-c/suppression could be reduced by removing T cells from the patients' marrow before the co-culture experiments; 4) patients' marrow T cells showed significant CFU-c/inhibition against autologous as well as against allogeneic unrelated normal marrow cells; 5) irradiation with 1500 rads of T cells abrogated their suppressor activity; 6) the latter was found mainly in marrow T cells with IgG receptors (TG+ cells) as well as in the supernatant of marrow T cells cultured overnight; 7) TG+ cells derived from normal donors had no significant CFU-c/inhibitory activity against autologous or allogeneic marrow cells. Active myelosuppression would thus be detectable in patients with SAA in remission and would appear to be mediated by marrow T cells with IgG receptors.